Many have sought ways to capture and harness the kinetic energy contained within the wind for generating power, such as generating electricity. Since about 1700 B.C., the windmill has been used for generating power, such as providing rotational energy to drive a machine or to pump/move water. More recently, numerous inventions and designs were developed, tested, and some actually used for generating electricity from the wind's kinetic energy.
The kinetic energy contained in a unit cross-section area of wind flow is somewhat limited, e.g., as compared to water flow. To commercially adopt a wind powered electricity generator typically requires a very large scale device. Up to now, the only wind powered turbine that has been relatively successful for commercially generating electricity is the wind impeller. A typical wind impeller 20 is shown in FIG. 1, for example. A book entitled Wind Power for Home & Business-Renewable Energy for 1990 and Beyond by Paul Gipe provides a detailed description about using wind impellers as wind powered generators, for example. Most or all other wind powered generator designs failed to be used commercially due to overly complicated or complex structures. Due to the complicated structures of other prior designs, such wind powered generators are typically difficult to build in large scale and/or too expensive to build in large scale.
Most or all wind powered generators may be grouped into one of two categories in terms of the aerodynamic mechanism used to capture the wind's energy and drive the wind powered generator. The first category includes the wind impeller type of wind powered generator (see e.g., impeller 20 in FIG. 1). Wind impellers typically have a blades that rotate about an axis generally aligned with the wind flow direction. Such rotation axis is usually horizontal. The blades are typically arranged in a vertical plane that is generally perpendicular to the wind flow direction, and each blade is tilted to some degree towards the wind. When the wind flows against and across the blades, the wind pushes the blades with a force component similar to the lift force component on a helicopter blade, similar to a lift force component on an airplane wing, and/or similar to a propulsion force component on an airplane propeller, but in an opposite force direction. Besides the wind speed, the magnitude of the “lift” force exerted on each blade depends on the angle of the blade relative to the wind, the aerodynamic shape (cross-section shape) of the blade, and the size of the blade. There are many shortcomings to an impeller type design for a wind powered generator, including low efficiency, high noise, danger of exposed spinning blades (e.g., hazardous to birds), space requirements, and difficulty in selecting a suitable blade material for a given climate, for example.
The second category of wind powered generators includes the wind turbine type with the rotational axis being generally perpendicular to the wind flow direction. Such wind turbines typically have flat blades, angled blades, or curved blades. The rotational axis may be horizontal (see e.g., U.S. Pat. Nos. 1,300,499, 1,935,097, 4,017,204, 4,127,356, 4,191,505, 4,357,130, and 5,009,569; and other country/region patents FR 2,446,391, FR 2,472093, DE 2,732,192, GB 2,185,786, and USSR 1,268,792) or vertical (see e.g., U.S. Pat. Nos. 2,335,817, 4,074,951, 4,076,448, 4,278,896, 4,350,900, 4,764,683, 5,038,049, 5,083,899, 5,332,354, 6,158,953, 6,191,496, 6,270,308, 6,309,172, and 6,538,340; and other country/region patents DE 2,505,954 and JP 1251), for example.
Wind powered generators of the second category may be divided into several groups based on the driving force of the turbine. In a first group, the drag force between the wind and the turbine blades exerts a driving force on the turbine for causing rotation. Such drag force depends on the velocity difference between the air passing over a blade and the turbine blade itself, as described by the following equation:γ=ηρair(Δu)2/2,where γ is the driving force of the wind turbine, η is the friction coefficient between the turbine blades and the air, ρair is the air density, Δu is the velocity difference between the air and the wind turbine blade. Because the friction coefficient is often a very small number, such wind turbines are not as efficient as wind turbines that uses the air lift force as the driving force (e.g., wind impeller shown in FIG. 1).
Such wind turbines using drag force typically have blades that are mostly or entirely exposed or blades installed in or partially covered by a wind conduit or wind tunnel structure with large gaps between the turbine blade ends and the interior walls of the air conduit (see e.g., U.S. Pat. Nos. 1,300,499, 2,335,817, 4,074,951, 4,191,505, 4,278,896, 4,357,130, 4,764,683, 6,191,496, 6,309,172, and 6,538,340; and other country/region patents DE 2,732,192, GB 2,185,786, and USSR 1,268,792).
A second group of the second category strives to use the maximum amount of the wind's kinetic energy. The primary driving force in the second group can be expressed as:γ=Δp,where Δp is the pressure difference between the front and the back of the wind turbine blade. In the second group, the wind turbine blades are installed in a conduit or shaped tunnel. Along the wind flow path through the turbine, the gap between the turbine blade ends and the wind conduit inside wall is minimized, so that the wind flow through such gap is negligible. The wind has to push the turbine blades to rotate the rotor before it flows out of the wind conduit. Examples of such wind powered generators are shown and described in numerous patents (see e.g., U.S. Pat. Nos. 1,935,097, 4,350,900, 5,009,569, 5,083,899, and 5,332,354; and other country/region patents FR 2,446,391 and FR 2,472093).
Theoretically, the driving force in this second group of turbines may be much greater than the lift force in the first category of turbines. When the turbine rotor is at rest, the driving force reaches the maximum (at certain blade positions), i.e., 100% of wind kinetic energy flowing through the wind conduit inlet area. This maximum driving force may be described by the following equation:γ=ρair(uw)2/2,where uw is the wind speed.
To manufacture and/or assemble a wind turbine with a minimized gap between the turbine blades and the wind conduit requires high standards of manufacturing quality to control the tolerances needed for minimizing the clearance gap. This leads to a third group of the second category. To avoid the difficulty and/or expense of minimizing the gap, many prior wind turbine designs of the third group are between or a combination of the first and second groups of the second category (see e.g., U.S. Pat. Nos. 4,017,204, 4,076,448, 4,127,356, 5,038,049, 6,158,953, and 6,270,308; and other country/region patents DE 2,505,954 and JP 1251). In the third group, many of the wind turbines also adopt some kind of wind funnel structure with varying (e.g, tapering) gaps. In such funnel structures, the gap between the interior walls of the funnel structure and the turbine blade ends is typically minimized at only one point or along a very short length of the wind flow path. Thus, the driving force on the turbine blades by the wind is a combination of drag force and pressure differential.
The blades of an impeller type of wind turbine (first category) completely face the wind to catch as much wind as possible. For the second category of the wind turbines, however, usually only half of the turbine blades are facing the wind. The blades on another half of such turbine normally rotate against the wind. Thus, the blades moving against the wind are often blocked from the wind (see e.g., U.S. Pat. Nos. 1,300,499, 1,935,097, 2,335,817, 4,017,204, 4,074,951, 4,127,356, 4,278,896, 4,357,130, 4,764,683, 5,009,569, and 6,270,308; and other country/region patents FR 2,446,391, FR 2,472093, DE 2,732,192, GB 2,185,786, and USSR 1,268,792). To improve the efficiency of the second category of wind turbines, many designs have been developed to change the wind flow direction so that more than half of the turbine blades can be pushed by the wind at a given rotational position of the rotor (see e.g., U.S. Pat. Nos. 4,076,448, 4,191,505, 4,350,900, 5,332,354, 6,158,953, and 6,309,172; and other country/region patents DE 2,505,954 and JP 1251). Very often, such designs involve very complicated structures. Hence, the cost of producing such designs is often too large, as compared to the electricity generated by such wind powered generators, and/or such designs are not feasible for a large scale machine.
Because the kinetic energy in unit cross-section area of a wind stream is very limited, many designs attempt to concentrate the wind energy by using a conduit with venturi shape (see e.g., U.S. Pat. Nos. 1,935,097, 4,017,204, 4,076,448, 4,127,356, 4,508,973, 4,963,761, 5,009,569, and 6,246,126; and other country/region patents FR 2,472,093, GB 2,185,786, and USSR 1,268,792). The inlet cross section area (perpendicular to the wind flow direction) of the wind conduit in such designs is usually much greater than the cross section area of the wind turbine at the rotor.